Automated painting systems normally involve spray booths traversed by a conveyor. Ventilation of the booths is always a prime consideration from two different points of view. The first is the prevention of the development of an explosive gaseous mixture in the booth area resulting from evaporation of paint solvents. The second concern is the confinement of toxic fumes and paint particulate so that the areas surrounding the booth are not seriously polluted. The latter problem has been handled by inducing an inflow of air into the spray booth through the openings traversed by the conveyor as it brings the workpieces into and out of the work area. Environmental concerns require that the gases exhausted from the booth be processed to remove toxicity before being dumped into the atmosphere. This can be done with a specially constructed incinerator, which may be considered as standard available equipment.
The practical side of all this is that it costs a considerable amount to provide and operate a blower system capable of handling enough air to meet the requirements for both prevention of explosion and area pollution, and to process the exhaust gases. The lower limit for an explosive mixture is about 2,000 cubic feet of air per gallon of evaporated solvent. Fresh air must be added to the booth in sufficient quantity to exceed this amount of air by a safe margin. This can be done by removing much less exhaust than the gaseous volume required to control area pollution. The latter problem requires that air be sucked into the booth in sufficient quantity to prevent any emergence of significant quantities of the evaporated solvent. The interplay of these two concerns has given rise to the principle of recirculation of some of the gases in the booth, and to reinject them into the booth at the openings to form part of the inflow stream required to confine the gases to the interior of the booth. The corresponding reduction in exhaust volume reduces the requirements on the exhaust fan, and on the exhaust processing system--both of these being expensive items that have costs increasing with increased capacity requirements. In summary, the practical side of the design of the ventilation system is to handle as little gas volume as you can without risk.
Special problems arise when the paint systems are adapted to handle relatively long, thin workpieces suspended from an overhead monorail conveyor in closely spaced relationship. Any rapid flow of gases tends to swing these pieces enough to cause them to collide as they move into, through, and out of the booth. These small collisions are enough to mar the painted surfaces (which have not yet set and dried), and cause rejection on later inspection. These conditions are often encountered in aluminum extrusion plants, where thin strips on the order of 20 feet long may be suspended about four inches apart along a conveyor moving at the rate of 15-20 feet per minute. Sometimes the pieces are slowly rotated by the conveyor to expose both sides to the spray system; and in other installations, the pieces are sprayed in successive booths, each operating on one side of the pieces. The pieces themselves can be in a cross-section that provides an airfoil effect, accentuating the tendency to swing in the presence of a cross or parallel draft. Minimizing the velocity of this draft is obviously critical. At these lower velocities, uniformity of velocity over the full vertical height of the opening is important to avoid areas where there may be inadequate inflow, or points of concentration of explosive mixture.